An insect's exoskeletal cuticle protects it from injuries and desiccation,as well as serving as an attachment site for skeletal muscles. As the cuticle is an incredibly hard-wearing structure, growth and development are only possible when it is shed and replaced by a larger one, a vital process known as ecdysis. Ecdysis is tightly regulated by various hormones that control the biosynthesis of essential proteins and initiate stereotyped behavioural patterns to remove the old cuticle. Although these hormones are conserved among moulting animals, their use varies across different taxa. In a recent paper published in Mechanisms of Development, a team of scientists led by Yoonseong Park from the Kansas State University report striking differences between the ways that beetles and flies moult.
20-Hydroxyecdysone triggers the onset of ecdysis, which is then orchestrated by various peptide hormones that regulate the physiological responses and behavioural patterns associated with ecdysis. Some of the most important ones are the eclosion hormone, ecdysis-triggering hormone (ETH),crustacean cardioactive peptide (CCAP) and bursicon. Working with the red flour beetle, Tribolium castanaeum, the team analysed the hormone's function during pupal-to-adult ecdysis. First they carefully determined the developmental expression pattern of the genes encoding the hormones as well as the hormones' cognate receptors. Then they went on to analyse the precise ecdysis behaviour, which consists of three phases.
Next the team took advantage of the insect's fully sequenced and annotated genome. Most importantly, the expression of particular target genes can be blocked very efficiently in Tribolium by RNA interference. This requires the injection of appropriate double-stranded RNAs into the small beetles, which needs a very steady hand. Fortunately Yasuyuki Arakane was the right man for the task, injecting dsRNAs for each of the target genes to identify the hormone's function.
The team found that almost all of the tested genes were absolutely essential for survival. The beetles either stopped developing at pre-ecdysis when the expression of the genes encoding eclosion hormone, ETH and the ETHR-A receptor were knocked-down, or at ecdysis when expression of the CCAP and CCAPR-2 receptor genes was impaired. Furthermore, they observed deficiencies after ecdysis when expression of the bursicon and bursicon receptor(rickets) genes was switched off. Only the insects that were kept from expressing the CCAPR-1 and ETHR-B receptors successfully completed ecdysis.
Comparing their results with data published for the fruit fly Drosophila the team concluded that ETH and eclosion hormone are necessary in Tribolium for pre-ecdysis and ecdysis behaviour, while eclosion hormone is not essential in the flies. Also, CCAP function seems to differ between the beetles and flies. While CCAP is necessary for ecdysis behaviour in Tribolium, Drosophila flies that lack CCAP show normal ecdysis. Possibly the most striking finding was that bursicon has a role in Tribolium postecdysial behaviour, but it is not required to tan the beetle's cuticle even though it is essential for tanning in Drosophila.
Studying the function of peptide hormones controlling ecdysis in Tribolium has revealed that conserved peptide hormones control ecdysis in different ways in different insect orders, yielding exciting insights into the evolution of this vital process. When it comes to ecdysis,it appears that nature is able to play different melodies using the same set of instruments.
